1. Field of the Invention
The present invention relates to the improved preparation of various catalysts, and, more especially, to the preparation of catalysts comprising the oxides of molybdenum and/or tungsten.
2. Description of the Prior Art
The subject catalysts are per se known to the prior art. They are used, in particular, for the preparation of .alpha.,.beta.-unsaturated aldehydes by the oxidation of olefins in the gaseous phase.
Thus, French Pat. No. 1,514,167 describes catalysts for the preparation of unsaturated aldehydes by the oxidation of olefins such as propylene and isopropylene either with air or oxygen, said catalysts having the general formula: EQU Ni.sub.a Co.sub.b Fe.sub.c Bi.sub.d As.sub.e P.sub.f Mo.sub.g O.sub.h
with a ranging from 0 to 20, b ranging from 0 to 20, (a+b) ranging from 0.5 to 20, c ranging from 0.5 to 8, d ranging from 0.1 to 7, e ranging from 0 to 3, f being less than 0.1, g being about 12, and h ranging from 36 to 98.
The aforesaid catalysts are prepared by adding an aqueous solution of suitable, water soluble salts of nickel, cobalt, iron and bismuth, a suitable arsenic compound and a suitable phosphorus compound, to an aqueous solution of a suitable molybdate, such as ammonium molybdate. The resulting paste is heated, on a support if desired, to evolve the water therefrom and to dry the cake which is formed.
The solid cake is then calcined at an elevated temperature, in air. The "suitable" water soluble salts noted in the aforementioned patent are nickel nitrate, cobalt nitrate, ferric nitrate and bismuth nitrate, for example.
And the French Pat. No. 1,604,942 features a process for the preparation of acrolein comprising the oxidation of propylene in gaseous phase, with a gas containing oxygen and water vapor, in the presence of a catalyst including the oxides of molybdenum, bismuth, iron and cobalt, the atomic content of which being as follows: Mo, 40.0 to 67.7%; Bi, 1.9 to 21.7%; Fe, 1.6 to 6.5% and Co, 21.0 to 48.1%.
The preparation of the catalysts described in this second patent consists of adding to a vigorously agitated aqueous solution of ammonium molybdate, an aqueous solution of the nitrates of iron, cobalt and bismuth. Subsequently, the water is eliminated and the resulting catalyst is calcined, optionally on suitable support.
Yet another process, described in published French application No. 2,147,933 (Ser. No. 72.20810), features the preparation of unsaturated carbonyl compounds from olefins. According to this particular process, the catalytic oxidation is effected in the presence of catalytic amounts of an oxide wherein the atomic ratios of the catalytic elements Co/Fe/Bi/W/Mo/Si/Tl/Z (Z=alkali or alkaline earth metals) are: 2.0 to 20.0/0.1 to 10.0/0.1 to 10.0/0.5 to 10.0/2.0 to 11.5/0.5 to 15.0/0.005 to 3.0/0 to 3.0, with the proviso that W+Mo is equal to 12.0.
These catalysts may be prepared by admixing aqueous solutions of ammonium molybdate and ammonium p-tungstate, adding to the mixture aqueous solutions of cobalt nitrate, ferric nitrate, bismuth nitrate, and thallium nitrate, together with an aqueous solution of a hydroxide or nitrate of either an alkali or an alkaline earth metal. The water is then evaporated therefrom and the resultant product calcined, optionally on a support.
It is also known to the prior art, from published French application No. 2,364,061 (Ser. No. 76.27531) to prepare a catalyst comprising the oxides of cobalt, molybdenum, bismuth and iron, and having the formula Co.sub.a M.sub.12 Fe.sub.b Bi.sub.c O.sub.x, with a ranging from 8 to 10, b ranging from 0.5 to 2, c ranging from 0.5 to 2 and x satisfying the various valences, whereby the active phase includes that phase corresponding to the formula Bi.sub.2 Mo.sub.2 Fe.sub.2 O.sub.12. Such catalysts result in markedly improved yields, for example, in the oxidation of olefins to .alpha.,.beta.-unsaturated aldehydes.
These catalysts are prepared, moreover, by interreacting an aqueous solution of ammonium heptamolybdate with an aqueous solution of Co, Bi and Fe nitrates. The water is then evaporated and the resulting paste dried. The solids obtained are subjected, optionally after a precalcination at approximately 450.degree. C., to a first calcination at a temperature ranging from 450.degree. C. to 500.degree. C., for at least 5 hours; then, after cooling to ambient temperature, a second calcination is conducted under the same conditions as the first.
The prior art is thus replete with references to the air or other oxidation of olefins, especially the oxidation of propylene to acrolein, utilizing catalysts comprising the oxides of cobalt, iron, bismuth and molybdenum. And a certain number of oxides of other metals have also been proposed as additives to, or adjuvants for, the base composition. It too is characteristics of the preparation of each of such catalysts that, in a first stage, ammonium heptamolybdate is reacted with the nitrates of iron, cobalt and bismuth.
During this reaction, ammonium nitrate is formed, which thermally decomposes at approximately 220.degree. C. during a subsequent calcination.
It is trivially apparent, though, that the thermal decomposition of ammonium nitrate poses certain very serious practical problems. For example, it is very well known to the art that ammonium nitrate is an explosive compound; therefore, its handling on an industrial scale is quite dangerous.
In certain processes, in place of the nitrates, the chlorides of the metals destined for incorporation in the catalyst desired, are also used. Accordingly, ammonium chloride, NH.sub.4 Cl, is then used, which during its thermal decomposition sublimes at about 340.degree. C. and is deposited in the form of a white powder in the apparatus. The resulting fouling of the equipment is very obviously harmful to the process for the manufacture of the catalyst.
It should be emphasized that on an industrial level it is extremely difficult, if not impossible, to eliminate the simultaneous or conjoint presence of ammonium salts and nitrates, or ammonium salts and chlorides.
Hence, a great need exists in this art for a process enabling the preparation of catalysts based, in particular, on the oxides of molybdenum and/or tungsten, and predicated upon the reaction of at least one ammonium salt and at least one chloride and/or nitrate of the topic metals, wherein there is no danger during any subsequent calcination operations of the thermal decomposition of ammonium nitrate and/or the sublimation of ammonium chloride.